This invention generally relates to protective coatings for components exposed to high temperatures. More particularly, this invention is directed to a coating system that provides sulfidation protection at elevated temperatures within a sulfur-containing environment, such as the hostile thermal environment of a gasification system used in gas turbine power generation plants.
Power generation plants exist that burn coal to produce coal gas or synthesis gas (SNG), which is then combusted in a gas turbine. The output of the gas turbine may be used directly to power an electric generator. In combined cycle gas turbine plants, the hot exhaust gases of the turbine are used to generate steam for powering a steam turbine. The production of coal gas occurs in what is commonly termed a gasifier.
The components of a coal gasifier, including injectors and nozzles, are subjected to a thermally and chemically hostile environment. For this reason, coal gasifier components may be coated with protective coatings. Common examples include oxidation-resistant coatings and thermal barrier coatings (TBCs). A particular example is a thermal barrier coating system comprising a ceramic coating bonded to the component surface with an oxidation-resistant metallic bond coat. Various ceramic materials have been used and proposed as TBCs, the most widely used being zirconia (ZrO2) partially or fully stabilized by yttria (Y2O3), magnesia (MgO), or ceria (CeO2). TBCs can be deposited by a variety of processes, including plasma spraying, flame spraying, and physical vapor deposition (PVD) techniques. The bond coat serves to promote adhesion of the TBC to the component. Common examples of bond coat materials include aluminum-rich compositions, for example, diffusion coatings such as diffusion aluminides and diffusion platinum aluminides, and overlay coatings of an MCrAlX alloy where M is typically iron, cobalt and/or nickel, and X is yttrium, rare earth elements, and/or reactive elements. These bond coat materials develop an aluminum oxide (alumina) scale as a result of oxidation, such as during deposition of the TBC on the bond coat as well as at high temperatures in an oxidizing environment. The alumina scale chemically bonds the TBC to the bond coat and provides environmental protection to the bond coat and underlying substrate.
While effective for providing oxidation resistance, traditional bond coat materials are not effective for protecting components from sulfidation in the high-temperature sulfur-rich environment of a coal gasifier.